Does low C-peptide during hyperglycemia indicate reduced endogenous insulin secretion?

In the assessment of established diabetes, the relationship between circulating glucose and C-peptide levels serves as a critical diagnostic window into pancreatic health. For a 70-year-old female, this metric is essential for distinguishing between insulin resistance and progressive secretory failure.

Clinical and physiological evidence

• C-peptide is co-secreted from pancreatic beta cells in equimolar amounts with insulin. Unlike insulin, which undergoes significant first-pass metabolism in the liver, C-peptide remains stable with a longer half-life, making it a highly accurate surrogate for endogenous insulin production.
• Under normal physiological conditions, hyperglycemia should trigger a robust secretory response. Low C-peptide levels (typically <0.2–0.6 nmol/L depending on the assay) in the presence of elevated glucose confirm absolute or relative insulin deficiency.
• In older populations, while renal impairment can lead to C-peptide accumulation and potentially mask early decline, a low C-peptide reading during hyperglycemia remains a definitive marker of failed beta-cell output.

Mechanistic insights

• The decline in insulin secretion in established diabetes is primarily driven by beta-cell dedifferentiation. Chronic glucotoxicity and metabolic stress cause mature beta cells to lose their identity and revert to a progenitor-like state.
• This process involves the downregulation of key transcription factors, such as PDX1 and MafA, which are essential for insulin synthesis and glucose-sensing mechanisms.
• Current research into "hub cells" suggests that metabolic failure often occurs because the most active cells are the first to succumb to oxidative stress. Even when a significant percentage of beta-cell mass remains, the functional exhaustion of these critical cells leads to the low-C-peptide phenotype observed in clinical settings.

Bottom line

Low C-peptide levels during hyperglycemia are a robust indicator of advanced pancreatic beta-cell dysfunction. This finding signals a transition toward insulin deficiency, necessitating a shift in therapeutic strategy to compensate for the loss of endogenous secretory capacity.